1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing a spark plug metal shell assembly including a metal shell and a ground electrode of a spark plug for use in a combustion device such as an internal combustion engine, and a method of manufacturing a spark plug.
2. Description of the Related Art
A spark plug is attached to, for example, a combustion device such as an internal combustion engine (engine), and is used to ignite an air-fuel mixture inside a combustion chamber. Generally, the spark plug includes: an insulating body which has a shaft hole; a center electrode which is inserted through the front end side of the shaft hole; a metal shell which is provided in the outer periphery of the insulating body; and a ground electrode which is provided at the front end portion of the metal shell and forms a spark discharge gap between the center electrode and the ground electrode. Further, a screw portion is formed in the outer peripheral surface of the metal shell so as to be threaded into an attachment hole of a combustion device when the spark plug is attached to the combustion device.
However, when the spark plug is attached to a combustion device, for example, the ground electrode is present between a fuel ejecting device and the spark discharge gap, and ejected fuel contacts the rear surface of the ground electrode. For this reason, mixed gas may not be sufficiently supplied to the spark discharge gap due to the presence of the ground electrode, and hence there is a concern that ignition performance may be degraded.
The disposition position of the ground electrode when the spark plug is attached to the combustion device may change depending on the relative positional relationship between the ground electrode and a thread ridge of the screw portion. Accordingly, in order to dispose the ground electrode at a desired position when the spark plug is attached to the combustion device, the thread ridge of the screw portion may be formed at a constant position in the ground electrode. Therefore, a method has been proposed which includes adjusting a position of a ground electrode in the circumferential direction by transporting a metal shell toward a rolling die used to form a screw portion by a rolling process while a ground electrode passes through a passage formed between a pair of guide plates, and forming the screw portion by the rolling die (for example, refer to Patent Document 1 or the like).
[Patent Document 1] Germany Registered Utility Model No. 202008015598
3. Problems to be Solved by the Invention
However, in the above-described method, the metal shell must be transported in a freely rotatable state in the circumferential direction in order to adjust the position of the ground electrode in the circumferential direction. For this reason, the metal shell rotates in the circumferential direction during a time when the ground electrode passes through the guide plates and the metal shell is disposed between the rolling dies. Hence, there is a concern that a positional deviation of the ground electrode may occur. Further, in the above-described method, the metal shell is moved toward the rolling die immediately before the rolling process, but there is a concern that the metal shell may rotate in the circumferential direction even during the movement. That is, in the above-described method, the positioning operation of the ground electrode may not be accurately performed in the circumferential direction, and then the relative positional relationship between the thread ridge of the screw portion and the ground electrode may become irregular.